This is our brooder - I call it the baby box

I guess it's about 3 1/2 feet by 4 feet and maybe 2 foot plus tall (not a real pull out the tape measure kinda gal) I actually just set the heat lamp on top and they were all good to go! They were about a week old when I brought them home and so far...knock on wood...we are 11 for 11 in the survivor department - so I guess it's part luck and part doing something right.
On top is the watch cat - she has learned to stay off the wire however!
Sounds mean - but until it happened I hadn't thought it out...she stepped on it right next to the light and it musta been hot (there is a 1/2 inch square hardware cloth wire cover on the top)! She howled and ran off to hide (I checked, no wounds, but she learned her lesson!) Now she prowls the edges and dreams of chicken dinners I am sure, but she stays off of the wire part.
Also, we have tile floors and it just has a thin wooden bottom - no issues - they move around when they need to. My sister warned me they would sleep in a pile - and they did at first - though they spread out more now. It was precious! But I would have been worried if she hadn't warned me! I guess it helps that it's by a window - in the morning I open the blinds and some wander over to the sunlight to bask in it. We have just started using the AC in the daytime and they aren't near any vents, but tend to nap under the lamp or in the sunshine if they are inside with me (I sleep in the daytime and work at night - so they do afternoon outings in the sun).
OH! I also keep the food and water on the far side away from the lamp - don't want to boil their water! More than likely you already had that figured out - but thought I would throw it in because it was a clever revelation for me

They seem to self adjust as needed - first couple of days mainly under the lamp or in the sunny part and now less and less of that. No expert, but figured I could share what I had learned. Good luck!